Do the hosts get detected by a normal discovery scan?
I'd create a new discovery scan and test again. If you don't get any results run the scan again and get a packet capture. Confirm that the scanner sending packets.
If it is sending packets you need to find out why your targets are not responding or not responding in a timely manner.
Yes, the hosts had been detected in an initial discovery scan but became inactive because no scan had been conducted against them.
After verifying some things, I now believe that it was the firewall protecting those servers which cause the problems because the source port being used by Foundstone to detect hosts is probably blocked.
Thank you guys